Introduce a case of corrosion problems caused by unreasonable pipeline design

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The last edit of this post was made by Wang Wei2 on 2024-1-3 at 11:27, introducing a case of corrosion caused by an unreasonable pipeline design. The chlorine used in the evaporator of a certain factory was sourced from two storage tanks. A large storage tank holds commercial chlorine, which contains no hydrogen chloride but has a small amount of dissolved water (about 30 ppm). The chlorine contained in the second large storage tank is produced from hydrogen chloride, with about 3.5% (mole percentage) hydrogen chloride, and very little water (less than 5 ppm). Both storage tanks and their outlet pipes are made of carbon steel, as this material is corrosion-resistant for such dry gas media. When designing the pipes leading into the evaporator, only one main pipe was installed for convenience. The outlet pipes of both storage tanks are connected to the main pipe. Just a few months after operation began, the main pipe behind the connection point suffered severe corrosion damage, resulting in a large amount of chlorine being released. Later, Hastelloy C-276 (a nickel-molybdenum-chromium alloy with excellent corrosion resistance) pipes were used as replacements, but their corrosion rate was also high. Analysis: This is a corrosion issue caused by unreasonable pipeline design. Although chlorine containing trace amounts of water, and dry chlorine containing a small amount of hydrogen chloride, have little corrosive effect on carbon steel, the corrosivity **increases** when oxygen, hydrogen chloride gas, and water vapor are present together. This is because the presence of hydrogen chloride gas reduces the solubility of water in chlorine, raises the dew point, and makes it easier for condensate to form. This corrosion problem is difficult to resolve from a material perspective, whereas changing the design can solve it effectively and cost-effectively. Improvement measure: The proper design involves installing an outlet pipeline from each of the two storage tanks directly to the evaporator, so that the two types of chlorine do not mix in the pipelines. Similar examples. The feed to a distillation column in a factory consists of fluorinated hydrocarbons, hydrogen chloride gas, trace amounts of hydrogen fluoride, and chlorine gas. The chlorine and hydrogen chloride emerging from the top of the tower are cooled in a brine condenser. After just a few weeks of use, the cold end of the condenser tubes (saltwater inlet) was corroded and damaged on the process side. It turns out that the inlet gas to the distillation tower contained trace amounts of water (a few ppm), which were not detected by the factory’s analysis instruments. The coexistence of chlorine gas, hydrogen chloride gas, and water vapor raises the dew point; water condenses at the lowest temperature points in the pipes, forming a highly corrosive substance that causes corrosion and perforation of the condenser tubes.